## Python’s len() Function: A Comprehensive Guide

### Introduction

Python programming has become increasingly popular in the software development industry due to its simplicity and versatility. One of the most essential functions in Python is the len() function.

In this article, we will explore the len() function and its various applications.

The len() function is a built-in function in Python, used to find the length of an object. Its purpose is to return the number of items in a sequence or collection.

This function is an attribute of a container object, and it returns an integer value representing the number of items in that object.

### Valid arguments for len() function

The len() function operates on sequence or collection objects. A sequence is an ordered and indexed collection of elements, and collections are a general grouping of elements.

Valid arguments for len() function include but are not limited to strings, lists, tuples, and range objects.

### Efficient use of len() function with container objects

When working with container objects, it is essential to be efficient in finding the length of the object using the len() function. The best approach is to use attribute access rather than iterating over the container object.

This is because attribute access is faster and less computationally expensive than iterations.

### Using len() with Built-in Sequences

#### Finding length of strings, lists, and tuples

The len() function can be used to find the length of strings, lists, and tuples.

For example, if we have a string variable named str_variable, we can find the length of the string as follows:

str_variable = "Hello, World!"

length_of_str_variable = len(str_variable)

print(length_of_str_variable)

This will print out the length of the string, which is 13.

#### Length of empty sequences

It is essential to determine the length of an empty sequence. An empty sequence is a sequence object that contains no items.

When a sequence is empty, the len() function will return a value of 0. For instance, if we have an empty list or string as follows:

empty_list = []

length_of_empty_list = len(empty_list)

print(length_of_empty_list)

empty_str = ""

length_of_empty_str = len(empty_str)

print(length_of_empty_str)

Both the empty list and the empty string will have a length of 0.

#### Obtaining length of range objects

A range object represents an immutable sequence of integers. It is used to generate a sequence of numbers within a given range.

When using the range function, we have to provide the start, stop, and step arguments. The len() function can be used to obtain the length of a range object.

For instance:

range_of_numbers = range(0, 100, 10)

length_of_range_of_numbers = len(range_of_numbers)

print(length_of_range_of_numbers)

This will print out the length of the range object, which is 10.

### Conclusion

The len() function in Python is a valuable tool for obtaining the length of a sequence or collection object. It is an attribute of a container object, and it returns the number of items in that object.

The len() function can be used on valid arguments, including strings, lists, tuples, and range objects. To be efficient, it is essential to use attribute access rather than iterating over the container object.

As we have seen, the len() function can be used to find the length of empty sequences and range objects. By mastering the len() function, programmers can write more efficient and effective code in Python.

### Using len() with Built-in Collections

#### Finding number of unique values in a list using sets

A set is an unordered collection of unique elements. One of the unique usages of set is in finding the number of unique values in a list.

For instance:

list_of_numbers = [1, 1, 2, 3, 3, 4, 5, 5, 5]

unique_numbers_set = set(list_of_numbers)

number_of_unique_numbers = len(unique_numbers_set)

print(number_of_unique_numbers)

This will print out the number of unique numbers in the list, which is 5.

#### Counting items in a dictionary

A dictionary is a collection of key-value pairs that can be used to store data in Python. The keys are unique and are used to access their corresponding values.

The len() function can be used to obtain the number of key-value pairs in a dictionary. For example:

number_count = {"one": 1, "two": 2, "three": 3}

number_of_pairs = len(number_count)

print(number_of_pairs)

This will print out the number of key-value pairs in the dictionary, which is 3.

### Exploring len() with Other Built-in Data Types

In Python, the len() function can only be used with specific data types that are iterable. These data types include strings, lists, tuples, sets, and dictionaries.

In this section, we will explore data types that are not iterable and hence incompatible with the len() function.

#### Inability to use len() with numbers and Boolean types

The len() function cannot be used to find the length of numbers and Boolean types. This is because numbers and Booleans are not iterable.

If we try to use the len() function on a number or a Boolean, we will receive a TypeError that states “object of type ‘int’ has no len().” For example:

number = 12345

length_of_number = len(number)

print(length_of_number)

This will raise a TypeError since we cannot get the length of an integer. Similarly, we cannot use the len() function with Boolean values as follows:

boolean_value = True

length_of_boolean_value = len(boolean_value)

print(length_of_boolean_value)

This will also raise a TypeError, stating “object of type ‘bool’ has no len().”

#### Inability to use len() with iterators and generators

Iterators and generators are built-in data types that generate and store a sequence of values on-the-fly. Since iterators and generators do not have a fixed length, we cannot use the len() function on them.

If we try to use the len() function on an iterator or a generator, we will receive a TypeError that states “object of type ‘generator’ has no len().” For instance:

number_generator = (number for number in range(10))

length_of_number_generator = len(number_generator)

print(length_of_number_generator)

This will raise a TypeError since we cannot get the length of a generator.

### Conclusion

In this article, we examined ways to use the len() function with built-in collections and explored data types that are not compatible with this function. We saw how to use the len() function to find the number of unique values in a list using sets and to count items in a dictionary.

We also learned how the len() function receives a TypeError when used on incompatible data types, including numbers, Booleans, iterators, and generators. By understanding the compatibility of the len() function with different data types, programmers can write more efficient and effective code in Python.

### Exploring len() Further with Some Examples

#### Ending a loop based on the length of a mutable sequence

Mutable sequences are objects whose values can be modified after creation. They include lists, sets, and dictionaries.

In some cases, we may want to end a loop based on the length of a mutable sequence that is being created by the user. For example:

user_inputs = []

while True:

user_input = input("Enter a value or type 'done' to end the loop: ")

if user_input == 'done':

break

user_inputs.append(user_input)

# End the loop if the user enters 10 items

if len(user_inputs) == 10:

break

In this example, we create an empty list called user_inputs and prompt the user to input values.

We check to see if the user wants to end the loop by checking if the input is the string ‘done’. If the user enters a value, we append it to the user_inputs list.

We also check the length of the list after each input and break the loop if the length of the list reaches 10.

#### Obtaining index of the last item in a sequence

Sometimes, we may want to obtain the index of the last item in a sequence. If the sequence is indexed, we can subtract one from the length of the sequence to obtain the index of the last item.

For example:

list_of_numbers = [1, 2, 3, 4, 5]

last_item_index = len(list_of_numbers) - 1

last_item = list_of_numbers[last_item_index]

In this example, we create a list of numbers and obtain the index of the last item by subtracting 1 from the length of the list. We then use the obtained index to access the last item in the list.

#### Splitting a sequence into two halves using the midpoint index

Sometimes, we may want to split a sequence into two halves using the midpoint index. We can obtain the midpoint index by dividing the length of the sequence by two.

For example:

list_of_numbers = [1, 2, 3, 4, 5]

midpoint_index = len(list_of_numbers) // 2

first_half = list_of_numbers[:midpoint_index]

second_half = list_of_numbers[midpoint_index:]

In this example, we create a list of numbers and obtain the midpoint index by dividing the length of the list by two using integer division ‘//’. We then use the midpoint index to split the list into two halves using slicing.

### Using len() Function with Third-Party Libraries

Third-party libraries are extensively used in Python programming for various purposes. Two of the most popular third-party libraries are NumPy and Pandas.

#### Using len() with NumPy ndarray data type

NumPy is a Python library used for scientific computing with multi-dimensional arrays. NumPy arrays are called ndarray, which are homogeneous arrays of fixed-size elements.

The len() function can be used to find the length of the first dimension of an ndarray. For example:

import numpy as np

array = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])

length_of_first_dimension = len(array)

print(length_of_first_dimension)

In this example, we create a NumPy ndarray and obtain the length of the first dimension by using the len() function.

#### Using len() with Pandas DataFrame data type

Pandas is a Python library used for data manipulation and analysis. A DataFrame is a two-dimensional table with rows and columns.

Pandas DataFrame is essentially a collection of Series instances, where each column represents a Series. The len() function returns the number of rows of the DataFrame.

For example:

import pandas as pd

data = {'name': ['Alice', 'Bob', 'Charlie'], 'age': [25, 30, 35]}

df = pd.DataFrame(data)

number_of_rows = len(df)

print(number_of_rows)

In this example, we create a Pandas DataFrame and obtain the number of rows by using the len() function.

### Conclusion

In this article, we explored some further examples of how to use the len() function in a variety of contexts. We saw how to end a loop based on the length of a mutable sequence that is being created by the user, how to obtain the index of the last item in a sequence, and how to split a sequence into two halves using the midpoint index.

We also examined how the len() function can be used with the popular third-party libraries NumPy and Pandas. By mastering the len() function and its usage with various data types, programmers can write more efficient and effective code in Python.

In conclusion, the len() function is an important built-in function in Python that is used to obtain the length of an object. We have explored several ways to use this function, including finding the length of built-in sequences, collections, and other data types.

We also discussed important considerations such as the inability to use len() with non-iterable data types. Among other examples, we explored how to obtain the index of the last item in a sequence, split a sequence into halves using the midpoint index, and use len() with third-party libraries such as NumPy and Pandas.

By mastering the len() function and its applications, programmers can write more efficient and effective code in Python. Remember, efficient use of the len() function can ultimately save time and prevent costly errors.